And if the surface is a generally smooth and glistening vision for anyone in a boat, below is a murky and sandstone boulder-studded slog, according to the underwater walker.

“It was like mountain climbing at times,” said Mr Moore, catching his breath after his six-hour walk. “The rocks were up to 10-feet high and covered in kelp.”

Having walked three kilometres under Lake Macquarie last year, Mr Moore thought he would trek along the bottom of an icon.

But the harbour journey, he said, turned out to be almost twice as long, more than double the depth, and immeasurably harder than the lake walk.

Mr Moore entered the harbour at Rose Bay on Wednesday morning and followed the shoreline for about five kilometres to Rushcutters Bay.

He said he was usually about six metres below the surface, but he reached depths of up to 15 metres.

As well as the boulders, Mr Moore had to step over rubbish, particularly bottles, but he said from his observations, the harbour was in generally good condition.

Rod Moore emerging from the water. Picture: Annette Holmes

Mr Moore was fed air through a line from a support boat. On board, changing air tanks and keeping an eye out for moorings and other boats, was Mr Moore’s mate, Mark “Snow” Boyce, from Caves Beach.

“It was pretty nerve-wracking at times,” Mr Boyce said. “We had a few hiccups, but we got there.”

At one point, Mr Boyce had to help free his friend’s air hose, after it was entangled on the keel of a yacht in Double Bay.

“It’s amazing how big those keels are,” Mr Moore said, adding that in places, visibility was very poor. “It’s dark down there on an overcast day.”

That darkness conjured up another worry: sharks. Mr Moore had swapped his home-made mesh contraption, which he wore on his back during the lake walk, for an electrical deterrent, Shark Shield, lent by the manufacturer. He saw no sharks.

However: “I saw shadows, but I was too busy watching my compass and trying not to look”.

Rod Moore waded out of Rushcutters Bay about 3pm, dragging a bicycle he had found on the bottom. He had undertaken the harbour walk to raise awareness of the Kiwanis International service clubs, but also for the sheer adventure of it.

Sydney Harbour walker Rod Moore. Picture: Annette Holmes

Now that he’s done it, Rod Moore reckons seeing the harbour from below is much better than observing it from a ferry or even the bridge.

“This is much more exclusive,” he said. “Anyone can go over the Harbour Bridge.”